A diffraction grating lens which has a diffraction grating provided on the surface of a lens is good for correcting for lens aberrations such as curvature of field or chromatic aberration (a shift in an imaging point depending on wavelength). This is because a diffraction grating has the idiosyncratic properties of inverse dispersion and anomalous dispersion, and has a great ability of correcting for chromatic aberration. When a diffraction grating is used for imaging optics, the same performance can be attained with fewer lenses than is possible with an imaging optics that is composed only of aspherical lenses. This provides an advantage in that the production cost can be reduced and that the optical length can be shortened, thus realizing a low profile.
With reference to FIGS. 18(a) to (c), a conventional method for designing the shape of a diffraction grating lens will be described. A diffraction grating lens is mainly designed by a phase function method or a high-refractive-index method. Herein, a designing method based on the phase function method will be described. The end result will also be the same when the high-refractive-index method is used for designing.
The shape of a diffraction grating lens is formed by combining the base shape of a lens body on which the diffraction grating is provided, i.e., a shape defining a refractive lens, and the shape of the diffraction grating. FIG. 18(a) shows an example where the lens body has a base shape Sb which is an aspherical surface, whereas FIG. 18(b) shows an example shape Sp1 of the diffraction grating. The diffraction grating shape Sp1 shown in FIG. 18(b) is determined by a phase function. The phase function is expressed by eq. (1) below.
                                          ϕ            ⁢                                                  ⁢                          (              r              )                                =                                                    2                ⁢                π                                            λ                0                                      ⁢                          ψ              ⁡                              (                r                )                                                    ⁢                                  ⁢                              ψ            ⁡                          (              r              )                                =                                                    a                1                            ⁢              r                        +                                          a                2                            ⁢                              r                2                                      +                                          a                3                            ⁢                              r                3                                      +                                          a                4                            ⁢                              r                4                                      +                                          a                5                            ⁢                              r                5                                      +                                          a                6                            ⁢                              r                6                                      +            …            +                                          a                i                            ⁢                                                r                  i                                ⁢                                                                  (                                                      r                    2                                    =                                                            x                      2                                        +                                          y                      2                                                                      )                                                                        (        1        )            Herein, φ(r) is a phase function which is represented by a shape Sp in FIG. 18(b); and Ψ(r) is an optical path difference function (z=Ψ(r)). r is a distance from the optical axis along a radial direction; λ0 is a design wavelength; and a1, a2, a3, a4, a5, a6, . . . , ai are coefficients.
In the case of a diffraction grating which utilizes first-order diffracted light, an annular zone is provided at every point where the phase from a reference point (center) reaches 2π in the phase function φ(r), as shown in FIG. 18(b). The shape Sbp of the diffraction grating plane shown in FIG. 18(c) is determined by adding the diffraction grating shape Sp1, which is based on the curve of the phase function being split every 2π, to the base shape Sb of FIG. 18(a).
In the case where the shape Sbp of the diffraction grating plane as shown in FIG. 18(c) is provided on an actual lens body, diffraction effects are obtained if the step height 141 of each annular zone satisfies eq. (2) below.
                    d        =                              m            ·            λ                                                              n                1                            ⁡                              (                λ                )                                      -            1                                              (        2        )            Herein, m is a design order (m=1 in the case of first-order diffracted light); λ is a wavelength used; d is a step height of the diffraction grating; and n1(λ) is the refractive index of a lens material which composes the lens body at the used wavelength λ. The refractive index of the lens material has wavelength dependence, and is a function of wavelength.
In any diffraction grating that satisfies eq. (2), there is a phase difference of 2π on the phase function between the foot and the edge of each annular zone, and, relative to light of the used wavelength λ, the optical path difference is an integer multiple of the wavelength. Therefore, the diffraction efficiency of first-order diffracted light relative to light of the used wavelength (hereinafter referred to as “first-order diffraction efficiency”) can be made approximately 100%. When the used wavelength λ changes, the value of d that makes the diffraction efficiency 100% will also change according to eq. (2). Conversely, if the d value is fixed, the diffraction efficiency will not be 100% at any wavelength other than the used wavelength λ that satisfies eq. (2).
In the case where a diffraction grating lens is used for generic imaging applications, there is a need to diffract light in a broad wavelength band (e.g., a visible light region spanning wavelengths of about 400 nm to 700 nm). Consequently, as shown in FIG. 19, when a visible light beam enters a diffraction grating lens having a diffraction grating 152 provided on a lens body 151, not only first-order diffracted light 155 which is ascribable to light of the wavelength that is selected as the used wavelength λ, but also diffracted light 156 of orders that are unwanted (hereinafter also referred to as “diffracted light of unwanted orders”) occurs. For example, if the wavelength which determines the step height d is a wavelength of green light (e.g., 540 nm), then the first-order diffraction efficiency at the green light wavelength will be 100%, so that no diffracted light 156 of unwanted orders will occur at the green light wavelength; however, the first-order diffraction efficiency will not be 100% at a red light wavelength (e.g., 640 nm) or a blue light wavelength (e.g., 440 nm), so that 0th order diffracted light of red or second-order diffracted light of blue will occur. These 0th order diffracted light of red and second-order diffracted light of blue are the diffracted light 156 of unwanted orders, which will spread across the image plane in the form of a flare or ghost, thus deteriorating the image or degrading the MTF (Modulation Transfer Function) characteristics. In FIG. 19, only second-order diffracted light is illustrated as the diffracted light 156 of unwanted orders.
As shown in FIG. 20, Patent Document 1 discloses providing an optical adjustment layer 161 which is composed of an optical material having a different refractive index and a different refractive index dispersion from those of the lens body 151, on the surface of a lens body 151 having a diffraction grating 152 formed thereon. Patent Document 1 discloses that, by prescribing specific conditions for the refractive index of the lens body 151 having the diffraction grating 152 formed thereon and the refractive index of the optical adjustment layer 161 formed so as to cover the diffraction grating 152, it is possible to reduce the wavelength dependence of diffraction efficiency, and suppress flare due to diffracted light of unwanted orders.
Patent Document 2 discloses, in order to prevent reflected light from the wall surfaces of the annular zones from being transmitted through the annular zone surfaces, providing light absorbing portions near the step feet of the annular zone surfaces. According to Patent Document 2, this structure can ensure that flare light reflected from the wall surface is not transmitted through the optical surface.
Patent Document 3 discloses a method of providing protrusions near the apices of annular zones of a diffraction grating so that the wavefront of spherical-wave light which is emitted from the annular zone surfaces is shaped into plane waves, thus improving the diffraction efficiency.